A multi-method and multi-model approach for predicting spatio-temporal patterns of sap flow, xylem isotopic composition, and water ages in the critical zone

Klaus, Julian; Chun, Kwok; Fabiani, Ginevra; Fresne, Maëlle; Hrachowitz, Markus; McGuire, Kevin; Moussa, Adnan; Penna, Daniele; Pfister, Laurent; Rodriguez, Nicolas; Schoppach, Remy; Sulis, Mauro; Zehe, Erwin

Abstract

Vegetation exhibits critical feedback with runoff generation. Trees show distinct water uptake patterns relying on soil water from different depths and groundwater with a mixture of water sources that is commonly very different from runoff in terms of age distribution and isotopic composition. Here we present a multi-method and multi-model approach to study the spatio-temporal patterns of sap flow and age distribution of tree water uptake and streamflow in a headwater catchment (mixed forest, 43 ha). For this, we monitored sap flow spatially distributed at >30 trees over two years, sampled 2H and 18O bi-weekly and spatially distributed in xylem for two years and in streamflow sub-daily for four years. This was supplemented by tritium sampling in streamflow over two years for different flow stages. We used statistical modeling to determine spatio-temporal patterns of transpiration and isotopic composition of xylem water and their drivers. We used a multi-model approach to derive catchment travel times through (i) composite Storage Selection (SAS) functions, (ii) conceptual hydrological modeling, and (iii) coupled land surface-subsurface modeling (ParflowCLM) combined with particle tracking. Statistical data analysis revealed that tree species, tree diameter, and topographic wetness index at the tree location were the main driver of spatial variability of sap flow, while soil water was the main source of xylem water with little groundwater influence. The travel time modeling showed a strong seasonal and event-based variability of travel times and allowed to include information on vegetation behavior with different complexity. Last, our detailed sampling of vegetation offers a blueprint for a sampling strategy of isotopes in xylem water for travel time studies. Our data revealed that approximately 20 sampled trees are sufficient to capture the mean isotopic value of xylem water of a species at our study site, while we needed around 100 samples to detect landscape influence on the xylem isotopes needed for considering spatial patterns in the travel time analysis. Our results underline the feedbacks between vegetation and runoff generation and show their relevance for better simulating catchment travel times.

• Presentation Conference Type: Presentation / Talk
• Conference Name: European Geosciences Union (EGU) General Assembly 2022
• Start Date: May 23, 2022
• End Date: May 27, 2022
• Deposit Date: May 26, 2022
• Publicly Available Date: May 27, 2022
• DOI: https://doi.org/10.5194/egusphere-egu22-1738
• Public URL: https://uwe-repository.worktribe.com/output/9572711
• Publisher URL: https://meetingorganizer.copernicus.org/EGU22/EGU22-1738.html